Photographic emulsions



Patented Jan. 16, 1945 UNITED STATES PATENT OFFICE 2,367,511 PHOTOGRAPHIG EMULSIONS ester, N. Y., assignors pany, Rochester, N. Y.,

Wesley G. Lowe and William F. Fowler, Jr. Rochto Eastman Kodak Coma corporation of New Jersey No Drawing. Application April 15, 1944, Serial No. 531,286 Claims. (Cl. 95-7) essing solutions under warm conditions to employ a hardener for the gelatin. By this hardening, the sticking of the emulsion to the hot ferrotyping roll is reduced or substantially eliminated and also processing of the emulsion is facilitated. In processing gelatin emulsions, it is often customary to incorporate hardening agents in the fixing solution to contribute to the hardness of the gelatin.

Recently, the use of polyvinyl alcohol or resins having a large proportion of polyvinyl alcohol has been suggested for photographic purposes, such as the carrier for a light-sensitive material. Silver halide emulsions have been referred to in which polyvinyl alcohol has been employed as the protective colloid therefor. For many purposes, however, polyvinyl alcohol or the polyvinyl acetates which are water-soluble by virtue of their polyvinyl alcohol content, have not been regarded as sufiiciently resistant to the effects of water or of elevated temperatures. For instance, with photographic prints where hot ferrotyping is desirable, some hardening is necessary to prevent adhesion of the emulsion to the heated surface. It does not follow, however, that any compound that has a hardening action upon polyvinyl alcohol or water-soluble poylvinyl acetate is suitable for use in a photographic emulsion. Some compounds may be satisfactory as hardeners of polyvinyl alcohol, and yet have a desensitizing or fogging action upon the photographic emulsion. Other materials, while not causing desensitization or haziness, are not sufficiently effective as hardeners to impart the desired resistance to hot rollers.

One object of our invention is to provide a method for hardening polyvinyl acetates having a content of polyvinyl alcohol sufficient to impart water solubility when used in photographic emulsions, so as to be readily susceptible to hot ferrotyping and also to be readily susceptible to processing by means of aqueous developing and fixing solutions. Another object of our invention is to provide a method for hardening photographic emulsions in which polyvinyl alcohol or watersoluble polyvinyl acetates are employed as the carrier without causing desensitization of those emulsions or affecting the clarity thereof. Other objects of our invention will appear herein.

Water solubility is ordinarily imparted to polyvinyl acetates by hydrolizing those compounds to the desired degree. By hydrolyzing off some of the acetyl, polyvinyl alcohol is formed in the compound, the polyvinyl alcohol imparting water solubility. In some cases, substantially all of the acetyl is hydrolyzed off, and the resulting product is termed polyvinyl alcohol in commercial usage, thisproduct ordinarily having approximately 5% polyviny1 acetate in the chemical combination. As a rule, polyvinyl acetates to apoint that they contain at least 45% polyvinyl alcohol exhibit a water solubility, although obviously the viscosity of the material may alter this limit in the individual cases. Our invention applies to polyvinyl acetates which have been hydrolyzed to a sufficient extent to convert sufficient of the polyvinyl acetate to polyvinyl alcohol so that water solubility is obtained. In usual practice, the range of polyvinyl alcohol content will be from 45-100% and will include not only materials known as partially hydrolyzed polyvinyl acetates having a polyvinyl alcohol content within this range but also materials which are known as polyvinyl alcohol and are either made up wholly of polyviny1 alcohol or have but a negligible proportion of polyvinyl acetate therein.

We have found that water-soluble reaction products of formaldehyde and melamine when prepared under desirable conditions cause photographic emulsions using the above-described polyvinyl compounds as the carriers therein to harden sufficiently so that processing and ferrotyping may be carried out without detrimentally affecting either the physical or photographic properties of the product so treated. We have found that the emulsions in accordance with our invention are susceptible to the action of the hardening agents without any detrimental action either on the emulsion or any of the materials contained therein, so that a clear, transparent, photographic material results. We have found that the combining of formaldehyde and melamine to form a satisfactory hardening material should be carried out under certain prescribed conditions to obtain the most advantageous hardening of the emulsion without desensitizing it. In some cases, desensitization is not objectionable or it might even be desirable. For instance, where an emulsion is desired which is sensitive to intense light but not to ordinary light, desensitization might be desirable. As an example of photographic emulsions which may be hardened in accordance with our invention, reference may be had to U. S. Patents No. 2,276,323 and No. 2,286,215 of Wesley G. Lowe.

' To obtain a hardening agent in which the desensitizing action is reduced to a minimum or entirely absent, it is desirable that the preparation of the hardener be carried out at a pH of 6 to 8 and preferably approximately 7. It is also desirable to use a mole ratio of formaldehyde to melamine of 3:1 in the reaction mass, to employ a temperature of 75-85 C. and to employ only a time necessary to cause the reaction to occur. For instance, it is desirable that the reaction be stopp d when the melamine goes into solution, although a time of up to 15 minutes is permissible when a reaction temperature of approximately 85 C. is employed. With a lower reaction temperature, a longer time beyond the dissolving of the melamine is permissible, although even in that case it is desirable to stop the reaction when the melamine is dissolved. The reaction is terminated by cooling the mass, such as to room temperature, after the melamine has dissolved. It is also preferred that the formaldehyde be employed in approximately 20% strength. If, as pointed out above, a desensitization action by the hardener is not objectionable, the conditions for the preparation of the formaldehyde-melamine hardener need not be as critical as when desensitization is objectionable. For instance, in its broader aspects, our invention contemplates the use of a melamine-formaldehyde hardener prepared at a temperature between 60' C. and 85 C., the time of heating being from to 2 hours after the melamine has dissolved and with a pH within the range of 3 to 9. Above or below this range of pH, there is a tendency toward formation of water-insoluble material. As it is desired that the hardener employed be completely water-soluble, the pH range given is preferred.

Also, where a hardener is desired regardless of the degree of desensitization which it may impart to the emulsion, one may go as far as :1 or even more in the mole ratio of formaldehyde to melamine in the reaction mixture, although with the higher proportions of formaldehyde to melamine waste of material may occur. Also, various aqueous dilutions of formaldehyde may be employed, such as 40%, 10%, 4%, or any concentration within the range of 40% down to 4% or less. Ordinarily, the more dilute solutions are more stable and, therefore, when the hardener is prepared using a high formaldehyde concentration it is desirable that it be diluted after its preparation, particularly if not intended for immediate use. It is thought that the hardener which is obtained is a mixture of methylolmelamines of various degrees of substitution such as at least 1, but less than 6 methylol groups attached to the triazine nucleus of the melamine through the nitrogen substituents attached to that nucleus.

The reaction described may even result in the formation of some hexamethylolmelamine. It is our opinion that this material is not as emcient for the hardening of the polyvinyl alcohol as the compositions described herein, and, therefore, while its presence in small amounts is not objectionable, it is preferable to so conduct the preparation of the hardener that the formation of hexamethylolmelamine is kept at aminimum. This is accomplished to the best advantage by conducting the reaction at a pH very close to 7.

In incorporating the desired hardener in a silver halide-emulsion, it is desirable that the pH of the emulsion be not allowed to exceed 7 to obtain the optimum hardening action. The hardening action of the melamine-formaldehyde hardener is comparatively slow, but a product is obtained which is not brittle but rather is tough and capable of withstanding the wear ordinarily met with in photographic emulsions.

The following example illustrates the preparation of a melamine-formaldehyde hardener having a good hardening action, good solubility in water and minimum desensitizing action on the photographic emulsion with which it is em loyed. Fifty cc. of a 20% formaldehyde solution ad- Justed to a pH of 7.0 was mixed with 12 grams of chemicalhr pure melamine and the mixture was heated rapidly on a steam bath. At C. the melamine dissolved in the formaldehyde. The pH'was adjusted to '1 by adding acetic acid, and the solution was in a form in which it could be employed immediately for hardening an emulsion of the type referred to herein. If it is necessary to store the hardening material, it may be stabilized by adding water thereto to dilute it to a volume of 200 cc.

The following examples illustrate the use of hardeners in accordance with our invention:

Emmple l A polyvinyl alcohol-silver halide emulsion was adjusted to a pH of 4.8 and an amount of the hardener described above was added so that the ratio of the weight of formaldehyde (used in making that amount of hardener) to polyvinyl alcohol in the emulsion was 4%. This emulsion was coated out and dried and allowed to stand for eight days. The emulsion was found to be able to withstand hot ferrotyping and the loss of photographic quality was little, if any.

Example 11 A polyvinyl alcohol-silver halide emulsion was adjusted to a pH of 5.2 and the described hardener was added in the following amounts, the percentages (by weight) being of formaldehyde (used in making the amount of hardener employed) to polyvinyl alcohol: 0.5%, 1%, 2%, 4%. It was found, after coating and drying, that with the 0.5% and 1% hardener values, although some hardening action was exerted on the emulsion, the emulsion did not harden to the point of withstanding hot ferrotyping. It was found, however, that by using 2% of hardener the material was adequately hardened in 30 days, while with 4% of hardener 20 days was sufilcient to harden the material to the point of withstanding hot ferrotyping without loss of photographic quality.

Example III A polyvinyl alcohol-silver halide emulsion was divided into six portions which portions were respectively given the following pHs: 3.7, 4.0, 4.3, 4.6, 4.9, 5.2.

To each portion 4% of the described melamine-formaldehyde hardener was added. After four days and twenty days the following results were obtained.

-'lhe terms complete" and 'A; complete refer to judgments based on the solubility of the emulsion in hot water. refers to a degree of hardness such that the emulsion did not melt-on commercial hot ferrotyping.

Example 4 Compositions were prepared of partially hydrolyzed polyvinyl acetates as follows:

Percent Percent l 1 a e t te In each case the colloid was dissolved in a solvent mixture of 1 part ethyl alcohol and 1 part water by volume. Besides the colloid, the solvent contained acetic acid or its salt and ethyl acetate, formed by the hydrolysis of the polyvinyl acetate and also a small amount of sodium sulphate which results from the neutralization of the sulphuric acid used as the catalyst in the polyvinyl acetate hydrolysis.

The following formula was employed for the preparation of photographic emulsions from A, B and C respectively:

30 grams of the solution of colloid were diluted with a mixture of 150 cc. of ethyl alcohol and 45 cc. of water. Into this solution held at 110 F. was run simultaneously solutions land 2 with rapid stirring:

Solution 1.l grams of silver nitrate dissolved in water and made up to a volume of 20 cc.

Solution 2.6.4 grams of potassium bromide, .25 gram of potassium chloride, .25 gram of potassium iodide, dissolved in water and made up to a volume of 20 cc.

To the dispersion so obtained were added 150 grams of the colloid solution. One hundred cc. of a 50% aqueous alcohol solution was added and the emulsion was sensitized, with 4 mg. of 2,3-

The term complete diethyl 4' methyloxathiazolo-carbocyanine iodide. Also 4% (based on the weight of resin present in the emulsion) of melamine-formaldehyde hardener was added.

The emulsions were then coated on paper and dried. After 5 days the paper was processed and was subjected to hot-ferrotyping. It was found that each of these emulsions passed through the processing baths unimpaired and that they resisted the meltin action of the hot-ferrotyping. The emulsions formed showed excellent photographic qualities.

The following formula was used jorpthe preparation of photographic emulsions from colloids D and E.

Solutions 1 and 2, as referred to above, were each diluted by'the addition of 20 cc. of ethyl alcohol to each. The procedur described above was otherwise followed. After the emulsion was sensitized, 8% of melamine-formaldehyde (based on the weight of resin in the emulsion) was added. The emulsions were coated out onto paper and they were found to be sufficiently hardened to process satisfactorily after 3 days and were resistant to the solvent action of hot water and to the melting action of hot-ferrotyping after 10 days.

The time of hardening of the emulsion in accordance with my invention decreases with 76 in a few days.

an increased proportion 01- the hardening agent. It is desirable'in practical operations that at least 4% of hardener based on the resin in the emulsion be employed to assure hardenin with- However, my invention contemplates polyvinyl acetate emulsions broadly having the vinyl acetate proportion specified with various proportions of melamine-formaldehyde hardener, as it is optional with the operator whether to use a proportion'of hardener which will give optimum properties within a few days or to use a smaller proportion of hardener and increase the time of storage before the emulsion is used. It is desirable to employ greater proportions of hardener with polyvinyl acetate emulsions having vinyl acetate contents in the higher portion of the specified range than with those emulsions in which the vinyl acetate content is in the lower part of the range. The percentage of hardener added throughout this 813-. plication refers to the ercentage by weight of formaldehyde used in making the hardener employed to the weight of polyvinyl acetate in the emulsion. Ordinarily, it is desirable that no more than 10% of the hardener be added to they emulsion. In cases where the amounts of formaldehyde, employed in preparing the hardener, are over 6 moles of formaldehyde to 1 mole of melamine, the ratio of formaldehyde to polyvinyl acetate in hardening the emulsion is figured on the basis or 6 moles of formaldehyde to 1 of melamine in preparing a hardener. In other words, in preparing a hardener where the mole ratio of formaldehyde to melamine is 6 or less the relation of hardener to polyvinyl acetate employed in making photographic emulsions is figured on the percentage of formaldehyde used in making the hardener to the polyvinyl acetate in the emulsion. When more than 6 moles of formaldehyde are employed to 1 of melamine in making the hardener, the percentage of hardener to polyvinyl acetate'in the emulsion is figured on the basis of 6 moles of formaldehyde to I of melamine in the hardener preparation.

The term water-soluble polyvinyl acetate" is employed in this application to refer to polyvinyl acetates having a polyvinyl alcohol content in sufficient amount to render the material water-soluble. This term includes materials which are referred to in commercial practice as polyvinyl alcohols.

I claim:

l. A photographic emulsion containing as the protective colloid for the photo-sensitive elementtherein a water-soluble hydrolyzed polyvinyl acetate and a water-soluble melamine-formaldehyde hardener resulting from reacting together one mole of melamine and 1 to 10 moles of formaldehyde.

2. A photographic emulsion containing as the protective colloid for the photo-sensitive element therein a water-soluble hydrolyzed polyvinyl acetate and a, water-soluble melamine-formaldehyde hardener resulting from reacting together 1 mole of melamine and 3 moles of formaldehyde.

3. A method of hardening a photographic emulsion containing water-soluble hydrolyzed polyvinyl acetate as the carrier. therein, which comprises mixing together 1 mole of melamine and 1 to 10 moles of formaldehyde in aqueous solution at a pH of 3.5-9 and a temperature of 6085 C. and 0 to 2 hours after the melamine dissolves, cooling the mass to below 60 C. followed by adding the mass to.the photographic emulsion.

4. A method of hardening a photographic emul- 5; A method or hardening a silver halide emullion in which water-soluble hydrolmd P yvinyl acetate is present as the protective colloid for the silver halide, which comprises mixing together l mole of melamine and l to 10 moles of iormnldohyde in aqueous solution at a pH of 3.5 to 9 and e temperature of 60-85 C. and 0 to 2 hour: after the melamine dissolves, cooling the mall to below 60 C. iollowed by adding the mute the 10 photozraphio emulsion.

WESLEY G. LOWE. WILLIAM F. mwum, Jl. 

